Emails are immutable, mailboxes are mutable - but only by deleting and writing a new email. Sure it can be similar to the previous email because copy and paste

Emails are immutable, mailboxes are mutable - but only by deleting and writing a new email. Sure it can be similar to the previous email because copy and paste exists - you're not hand scribing it. But the fact remains that for the mailstore, this is a brand new message minted out of whole cloth. You can't edit in place. And in terms of how our own servers work - there is a backup of the old version, because it was a separate message in its own right that was deleted by the action of making a new message and deleting the old one.